Recent advances in manufacturing, communications, sensors, electronics, and control technologies have witnessed an unprecedented application increase of unmanned aerial vehicles (UAVs) in civilian and commercial domains, such as monitoring and detection, traffic control, emergency search and rescue, emergency communication recovery.
Among these applications, UAVs are responsible for transmitting high-speed endogenous data (sensed by onboard equipments) or exogenous data (received from other relays or base stations (BSs)) to a ground control station (GCS) or ground communication users. However, UAVs have stringent size, weight, and power constraints; therefore, energy-efficient data transmission should be closely considered in UAV communications.
Besides, the safety requirements are paramount. UAVs are obligated by the law to stay in the remote control range of a controller to ensure the safe flight. To this aim, control links for ultra-reliable and low-latency communications (URLLC) should be enabled [1]}, [2]}.
In [1]}, the average achievable data rate for URLLC-enabled UAV systems was investigated, where a URLLC control link was established to deliver control information to a UAV for collision avoidance. The problem of how to optimize transmission distances and the packet blocklength in URLLC aided multi-hop UAV relay links for short packet transmission was studied in [2]}
Additionally, owing to service requirements among these applications, a UAV may fly in a three-dimensional (3D) trajectory, which brings great challenges for air-ground propagation channel modeling. In 3D space, the air-ground channel model depends not only on a UAV's horizontal location, but also its deployment altitude.
In [5]}, the statistical behavior of the path-loss from a BS to a UAV was modelled.
Measurement-based modeling and analysis of UAV channels in the vertical dimension were performed in [6]}
Besides, the work in [7]} explored artificial neural networks (ANNs) to generate numerical UAV channel coefficients. However, the channels obtained in [5]}, [6]}, [7]} are either statistical results that can only simulate channels in an average sense or numerical results without proposing theoretically analytical UAV channel models.
